LONDON— Royal Dutch Shell PLC bet big on natural gas as the energy source of the future when it bought BG Group for $54 billion. Five years later, it appears the gas era won’t last long.

Falling prices for wind and solar power, coupled with government and businesses’ new green goals, are accelerating a shift to cleaner energy and leaving natural gas—long seen by energy companies as a bridge between fossil fuels and renewables—in the lurch. The fuel is also under growing scrutiny for methane leaks, leading some potential customers to skip gas and move ahead to lower-carbon alternatives.

That is a risk for Shell and rivals such as Exxon Mobil Corp. and Total SE , which also invested in gas, given that gas projects typically cost billions of dollars up front and take decades to recoup that investment.

Shell last month halved its outlook for global gas demand growth to 1% a year, and said demand for the fuel could peak as soon as the 2030s.

While burning gas emits less greenhouse gas than coal does, environmental gains are lost if there is a leakage of methane, the main component of natural gas. Methane is more potent than carbon dioxide in contributing to climate change and has become a target of environmentalists.